The goal of this research is to discover the structural features of sensory neurons that define their capacities to code information. The emphasis is on the cytological design and pattern of connections of specific types of auditory neurons and their synaptic contacts at several levels of the brain stem and cochlea. This study relies on the Golgi and classical silver techniques to demonstrate neuronal architecture, silver-degeneration, histochemical, and autoradiographic methods to trace connections, and electron microscopy applied to normal and experimental material. Sufficient details are obtained to correlate with electrophysiological recordings of sensory neurons and with electron microscopic observations of sensory synapses. The resulting relations provide a basis for exploring the mechanisms accounting for stimulus coding in the auditory system.